Fuchs endothelial corneal dystrophy (FECD) is the most common and clinically relevant inherited corneal disease in the US population and the third leading cause of corneal transplants in 2007. FECD results from chronic loss of corneal endothelial cells which line the inner surface of the cornea and maintain physiologic hydration necessary for corneal clarity. In some cases, FECD is caused by missense mutations in the alpha 2 collagen VIII (COL8A2) gene. Very little is known about the pathogenesis of FECD due to the fact that only end stage corneas obtained at corneal transplant are available for study. Limited work indicates that apoptosis is occurring in endothelial cells from advanced FECD corneas, although the mechanisms leading to apoptosis are poorly understood. Thus, animal and cell culture models would be invaluable tools to elucidate early events leading to endothelial cell apoptosis in FECD. Our preliminary work includes: 1) identification of markers for the unfolded protein response (UPR) in the endothelium of FECD patient corneas. Proper protein folding is critical for normal function of the cell. The UPR is a highly conserved, broad ranging cellular response induced by an excess of misfolded proteins. Initially, the UPR functions to relieve stress of unfolded proteins through selective alterations in transcription and translation. If protein folding demand and capacity remain unbalanced, the UPR induces cells to undergo apoptosis. In addition, 2) we have developed two gene targeted knock-in mouse lines with different COL8A2 mutations known to cause FECD in humans. Preliminary results show pathologic changes in the basement membrane of the corneal endothelium which are highly similar to those seen in human FECD corneas. The overall hypothesis of this application is that corneal endothelial cell expression of COL8A2 mutations activates the unfolded protein response and results in endothelial cell death. The availability of COL8A2 FECD knock-in mice with a relevant pathologic phenotype provides an invaluable opportunity to test this hypothesis as follows: Aim 1 will assess COL8A2 mutant mice for corneal endothelial cell apoptosis and will characterize in greater detail pathologic changes in corneal endothelium and the endothelial basement membrane comparable to human FECD corneas. Aim 2 will assess for UPR activation in corneas and cultured corneal endothelial cell lines from COL8A2 mutant mice. Aim 3 will assess for synergistic effects of UV light (a known inducer of the UPR and environmental stressor for the corneal endothelium) on UPR activation in corneas and cultured corneal endothelial cell lines from COL8A2 mutant mice. This proposal seeks to use COL8A2 FECD mutant mice and corneal endothelial cell lines to establish a novel mechanistic link between the UPR, a major cell stress pathway, and corneal endothelial cell apoptosis resulting from FECD mutations in COL8A2. The establishment of such a link would represent a major advance in the National Eye Institute's goals of improving knowledge and treatment of inherited corneal diseases.